In wireless communications systems, designers often grapple with the difficult problem of reliably transmitting a signal through a complex and dynamic environment. Fixed obstacles, such as buildings, streets or walls reflect and refract transmitted signals in varying amounts, causing elements of the signals to be distorted, separated, phase-shifted or delayed. Dynamically moving obstacles, such as automobiles, bicyclists, and pedestrians further complicate the environment, or transmission channel. Consequently, after being transmitted from a source location, multiple copies of the same signal may be received at different times, at different phases, and with differing distortions at a single receiver, depending on the path each respective signal traversed through the transmission channel. These undesirable properties force designers to make various trade-offs among signal quality, propagation delay, channel capacity, amplification, frequency and error correction requirements.
In an effort to increase channel capacity, designers have implemented systems which employ multiple antennas in both transmitters and receivers. Typically, the antennas are spaced apart as an array at both transmission and reception locations. Each antenna within its respective array is generally configured to maintain a specific gain and phase relationship with the other antennas within the array. These gain and phase relationships are typically maintained by weighting the signal, prior to transmission, with an appropriate weighting vector. In a properly configured transmitter array, the end result is that the array produces a transmission pattern that is more focused on a given receiver than that which could be produced by a comparable single antenna.
Antenna arrays were previously used to improve signal quality. Use of antenna arrays at both the transmitter and receiver has recently been proposed to increase channel capacity. When multiple antennas are used at the transmitter and receiver, the wireless channel between them may be referred to as a multiple-input, multiple-output (MIMO) channel. MIMO systems rely on the existence of multipath propagation between a transmitter and a receiver. Individual beams, pointing in different directions and carrying different traffic, can be formed at a multi-antenna transmitter. In addition, individual received beams, carrying different traffic and arriving from sufficiently different angles, can be separated at the multi-antenna receiver through a combination of nulling and subtraction.
To date, most MIMO systems have been constructed to minimize the processing power required at the mobile unit (which include, for example, cellular telephones). Doing so allows the mobile units to be smaller, more power efficient and less expensive than they might otherwise be. Consequently, system designers have typically kept the processor-intensive and power consuming mechanisms required to form and manage individual data channels at the base station, where power supplies are significantly greater and where the benefits of such systems outweigh their costs.
Base station only processing requires the base station to have Channel State Information (CSI) describing the state of the communications channel between the base station and the mobile unit. In cellular systems, for example, where the forward and reverse channels are typically confined to different frequency bands, the CSI must be forwarded to the base station from the mobile units, which then requires some increase in processing and power consumption at the mobile units. Techniques such as Minimum Mean Squared Error (MMSE), Transmit Zero Forcing, and the use of Filter Banks have been used to address these problems, but all have met with limited success.
Although MIMO systems frequently result in increased cost and complexity, MIMO systems have numerous advantages over their traditional counterparts. Perhaps most notably, MIMO systems are able to dramatically increase the data throughput rate of a given channel without any associated increase in bandwidth or constellation order. This allows more information to be transmitted through the same channel, which facilitates clearer voice communications, higher data throughput, and more reliable wireless transmissions.
Despite their advantages, however, existing MIMO systems have also been burdened with significant disadvantages. In a typical MIMO system, for example, multiple signals are transmitted from multiple antennas at the same time and at the same frequency. In these systems, the transmitted signals will tend to interfere with each other, resulting in cross-talk among wireless communication paths or channels. This interference is undesirable, and leads to signal distortion, phase shifting or even cancellation. Furthermore, in existing implementations, signal-to-noise ratios for individual data streams transmitted between a MIMO transmitter and MIMO receiver may be substantially different, resulting in highly different bit error rates, and creating additional difficulties in transmission.